Reducing latency of memory read operations returning data on a read data path across multiple clock boundaries, to a host implementing a high speed serial interface

ABSTRACT

A calibration controller determines a latest arriving data strobe from at least one data chip at a first data buffer in a read data path between at least one memory chip and a host on a high speed interface. The calibration controller determines whether external feedback of the at least one data chip is required. The calibration controller, in response to determining that external feedback of the at least one data chip is required, aligns a chip clock distributed to a second data buffer in the read data path with the latest arriving data strobe by applying a 180 degree phase align of the chip clock through one or more latches, wherein data cross a first clock boundary from the first data buffer to the second data buffer, to minimize a latency in the read data path across the first clock boundary.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of commonly assigned U.S. patentapplication Ser. No. 15/866,838, filed Jan. 10, 2018, which is herebyincorporated herein by reference.

BACKGROUND 1. Technical Field

This invention relates in general to a memory system calibration andmore particularly to reducing latency of memory read operationsreturning data on a read data path across multiple clock boundaries, toa host implementing a high speed serial interface.

2. Description of the Related Art

Computing systems generally include one or more circuits with one ormore memory or storage devices connected to one or more processors viaone or more controllers. Timing variations, frequency, temperature,aging and other conditions impact data transfer rates to and from memoryor other storage, which impacts computer system performance. Inaddition, in a computer system where a host implements anserializer/deserializer (SerDes) based, high speed serial (HSS)interface for interfacing between a memory device operating under afirst memory protocol and a host that is agnostic to the first memoryprotocol, for asynchronous read operations, timing variations betweenclock and data signals along a read data return path have the potentialto significantly impact timing margins within the computer system.

BRIEF SUMMARY

In one embodiment, a method is directed to determining, by a computersystem, a latest arriving data strobe from at least one data chip at afirst data buffer in a read data path between at least one memory chipand a host on a high speed interface. The method is directed todetermining, by the computer system, whether external feedback of the atleast one data chip is required. The method is directed to, in responseto determining that external feedback of the at least one data chip isrequired, aligning, by the computer system, a chip clock distributed toa second data buffer in the read data path with the latest arriving datastrobe by applying a 180 degree phase align of the chip clock throughone or more latches, wherein data cross a first clock boundary from thefirst data buffer to the second data buffer, to minimize a latency inthe read data path across the first clock boundary.

In another embodiment, a computer system comprises one or moreprocessors, one or more computer-readable memories, one or morecomputer-readable storage devices, and program instructions, stored onat least one of the one or more storage devices for execution by atleast one of the one or more processors via at least one of the one ormore memories. The stored program instructions comprise programinstructions to determine a latest arriving data strobe from at leastone data chip at a first data buffer in a read data path between atleast one memory chip and a host on a high speed interface. The storedprogram instructions comprise program instructions to determine whetherexternal feedback of the at least one data chip is required. The storedprogram instructions comprise program instructions to, in response todetermining that external feedback of the at least one data chip isrequired, align a chip clock distributed to a second data buffer in theread data path with the latest arriving data strobe by applying a 180degree phase align of the chip clock through one or more latches,wherein data cross a first clock boundary from the first data buffer tothe second data buffer, to minimize a latency in the read data pathacross the first clock boundary.

In another embodiment, a computer program product comprises one or morecomputer-readable storage devices and program instructions, stored on atleast one of the one or more storage devices. The stored programinstructions comprise program instructions to determine a latestarriving data strobe from at least one data chip at a first data bufferin a read data path between at least one memory chip and a host on ahigh speed interface. The stored program instructions comprise programinstructions to determine whether external feedback of the at least onedata chip is required. The stored program instructions comprise programinstructions to, in response to determining that external feedback ofthe at least one data chip is required, align a chip clock distributedto a second data buffer in the read data path with the latest arrivingdata strobe by applying a 180 degree phase align of the chip clockthrough one or more latches, wherein data cross a first clock boundaryfrom the first data buffer to the second data buffer, to minimize alatency in the read data path across the first clock boundary.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The novel features believed characteristic of one or more embodiments ofthe invention are set forth in the appended claims. The one or moreembodiments of the invention itself however, will best be understood byreference to the following detailed description of an illustrativeembodiment when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a block diagram illustrating one example of a system includingmultiple clock boundaries between at least one memory buffer system anda host, across an HSS interface;

FIG. 2 is a block diagram illustrating one example of a memory buffersystem configured in a distributed memory buffer topology with adedicated connection to an HSS interface;

FIG. 3 is a block diagram of one example of a memory buffer systemconfigured in a unified memory buffer topology with a dedicatedconnection to an HSS interface;

FIG. 4 illustrates a block diagram of one example of a generalimplementation of a DDR read path of an HSS interface within a memorybuffer system;

FIG. 5 illustrates a timing diagram of one example of an ideal clock andstrobe alignment compared with a run time clock and strobe misalignmentto be minimized by a calibration controller to minimize read datalatency between a memory buffer system and a host, across an HSSinterface;

FIG. 6 illustrates a block diagram of one example of a DDR read path ofan HSS interface optimized for multiple memory buffer chips in amulti-port system;

FIG. 7 illustrates a block diagram of one example of external feedbackcontrol of a PLL;

FIG. 8 is a block diagram illustrating one example of a computer systemin which one embodiment of the invention may be implemented; and

FIG. 9 illustrates a high level logic flowchart of a process andcomputer program for optimizing a read data path between one or morememory buffer chips of a memory buffer system operating under aparticular memory protocol and a memory protocol agnostic host employingan HSS interface.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth to provide a thorough understanding ofthe present invention. It will be apparent, however, to one skilled inthe art that the present invention may be practiced without thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form to avoid unnecessarily obscuring thepresent invention.

In addition, in the following description, for purposes of explanation,numerous systems are described. It is important to note, and it will beapparent to one skilled in the art, that the present invention mayexecute in a variety of systems, including a variety of computer systemsand electronic devices operating any number of different types ofoperating systems.

FIG. 1 illustrates a block diagram of a system including multiple clockboundaries between at least one memory buffer system and a host, acrossan HSS interface.

In one example, a system 100 may include one or more memory buffersystems, such as a memory buffer system 110, connected to one or morehosts, such as a host 114, implementing one or more SerDes basedconnections, such as an HSS channel 160, connected to a HSS interface112 of memory buffer system 110. In one example, memory buffer system110 may represent a disparate memory system from host 114. For example,memory buffer system 110 may implement a synchronous double data rate(DDR) protocol and host 114 may represent a device that is agnostic toany particular memory protocol and employs HSS channel 160 to controldata transfers between one or more memory chips and host 114. In oneexample, HSS channel 160 may represent multiple differential high-speeduni-directional channels. In one example, HSS interface 112 mayserialize data received from a parallel interface of memory buffersystem 110, for access by HSS channel 160 of host 114. In one example, aSerDes connection implemented in HSS interface 112 and HSS channel 160may represent one or more pairs of functional blocks which may be usedin high speed communications to convert data between serial datainterfaces and parallel interfaces in each direction. In one example,HSS interface 112 may include one or more of a parallel in serial out(PISO) block and a serial in parallel out (SIPO) block, configured inone or more different architectures, incorporating one or more types ofclocks. In one example, HSS interface 112 may provide data transmissionto HSS channel 160 over a single line to minimize the number of I/O pinsand interconnects required for an interface.

In one example, memory buffer system 110 may control multiple dynamicrandom-access memory (DRAM) devices 108, which may also be referred toas memory chips. One or more memory buffer chips, such as memory bufferchip 106 of memory buffer system 110, may be configured in one or moretopologies connected to DRAM device 108, including, but not limited to adistributed memory buffer topology, such as a distributed memory buffertopology as illustrated in FIG. 2, and a unified memory buffer topology,such as a unified memory buffer topology as illustrated in FIG. 3. Inone example, as described herein, DRAM devices 108 may generally referto one or more types of memory including, but not limited to,traditional DRAM, static random access memory (SRAM), and electricallyerasable programmable read-only memory (EEPROM), and other types ofnon-volatile memories. In one example, memory buffer system 110 mayrepresent one or more dual in-line memory modules (DIMMMs) or registereddual in-line memory modules (RDIMMs) implementing double data rate (DDR)synchronous DRAM (SDRAM), such as, but not limited to, DDR type three(DDR3) SDRAM and DDR type four (DDR4) SDRAM. In one example, memorybuffer system 110 may represent one or more of, or a combination of, oneor more integrated circuits (ICs), one or more application specific ICs(ASICs), and one or more microprocessors. In additional or alternateexamples, memory buffer system 110 may represent any type of system thattransmits data bidirectionally or unidirectionally between a controllerand a chip.

Synchronizing input signals from one chip clock domain to another chipclock domain between memory buffer system 110 and host 114 may requireone or more asynchronous boundary crossings. The asynchronous boundarycrossings may increase the latency in a path, which decreases theperformance of systems with chip crossings. For example, memory buffersystem 110 may include a dedicated connection through memory buffer chip106 to SerDes based, HSS interfaces, such as HSS interface 112, forconnecting to a host that employs an HSS interface, such as HSS channel160 of host 114. In one example, to accommodate for read requestresponses to asynchronous read requests through the dedicated connectionfrom memory buffer system 110 to host 114 through HSS interface 112,system 100 may implement a DDR read path 140 that includes additionaldata buffers for gating data and other components along the data pathbetween memory buffer system 110 and host 114 through HSS interface 112.In particular, the additional data buffers may be implemented tocompensate for skews in phase alignment between clock signals and datasignals within memory buffer system 110.

In one example, the additional data buffers implemented in the dedicatedpath of HSS interface 112 may include first in first out data buffers,such as a memory FIFO 130, of memory buffer system 110, and a transmit(TX) FIFO 132, of HSS interface 112. In one example, memory FIFO 130receives data sampled by memory buffer system 110 in a DDR protocol,such as DQ data sampled by DQS strobe signals, in a first clock domain.The data from memory FIFO 130 may pass across a clock boundary 120 ofthe first domain, through an internal data path 131 to TX FIFO 132,operating in a second clock domain. The data from TX FIFO 132 may passacross a clock boundary 122 of the second clock domain, to a serializer134, operating in third clock domain. In the example, to compensate forthe phase misalignment between clock signals and data signals of memorybuffer system 110, DDR read path 140 may include one or more elements tosynchronize the data as the data cross clock boundary 120 and clockboundary 122. For example, internal data path 131 may add one or moreadditional cycles between a read pointer reading data from memory FIFO130 into TX FIFO 426 and serializer 134 may add one or more additionalcycles between data being loaded into memory TX FIFO 426 and an unloadpointer reading data out of TX FIFO 426 into serializer 134. In oneexample, serializer 134 may convert the data received in a DDR protocolinto a protocol for transmission on a HSS channel 160, and place theconverted data on HSS channel 160 for access by host 114.

In the example, in transferring data from memory buffer system 110 tohost 114, the transfer of data is illustrated crossing two separateclock boundaries, illustrated by clock boundary 120 and clock boundary122, where each clock boundary may represent a transition across a databuffer controlled by one clock signal, in one domain, into another clockdomain, controlled by a different clock signal. In additional oralternate embodiments, the transfer of data from memory buffer system110 to host 114 may cross additional clock boundaries, such as three ormore separate clock boundaries.

In the example, the addition of data buffers, and data passing through aclock boundary at each data buffer in DDR read path 140 in HSS interface112, introduces additional latency into the overall latency of the readdata return path. The additional latency may increase significantly inthe event of any non-optimal clock alignment between the clockboundaries that necessitates stalling data in each data buffer for oneor more clock cycles. In one example, the overall system latency of areturn data path may be measured as a function of how long host 114waits, after sending an asynchronous read request, for a read tagresponse from memory buffer system 110, where subsequent to host 114receiving the read tag response, the corresponding data are guaranteedto return N cycles later. While memory buffer system 110 may becontrolled under a timing protocol, such as a Joint Electron DeviceEngineering Council (JEDEC) timing protocol, where within memory buffersystem 110, data and control latency relationships are required to bemaintained across data buffer chips, in the present invention, data aretransmitted outside the boundaries of the memory protocol controllingmemory buffer system 110 to a memory protocol agnostic host and arerequired to traverse multiple FIFOs, over multiple clock boundaries, toarrive at host 114, introducing additional latency to the end-to-endread data path. In one example, the additional latency introduced bytransferring data across each clock boundary may include multiple memoryclock cycles, which accumulates to an amount of delay that appreciablyimpacts performance of read accesses from memory buffer system 110 byhost 114.

In the present invention, the overall system performance of system 100may be optimized by minimizing the read data latency from memory buffersystem 110 to host 114 through DDR read path 140 of HSS interface 112.In one example, the overall read data path latency, from memory buffersystem 110 to HSS channel 160 of host 114, via HSS interface 112, may beminimized by optimizing the wait time from when host 114 sends a readdata request to the cycle that host 114 receives a read tag response,across multiple clock boundaries, such as clock boundary 120 and clockboundary 122. In particular, in the present invention, the overall readdata path latency may be minimized by optimizing the intermediate datatransfers through memory FIFO 130 and TX FIFO 132, along internal datapath 131, to serializer 134, allowing the read tag response to bepotentially returned one or more cycles earlier than if no optimizationis performed. By optimizing the intermediate data transfers throughmemory FIFO 130 and TX FIFO 132, the overall system latency of memoryread operations across multiple clock boundaries is reduced, therebyminimizing read data latency within system 100.

In the present invention, the memory controller and associated memoryinterface maintenance and calibration functions may be initiated andcontained within memory buffer system 110. For example, a calibrationcontroller 150 may manage calibration and training functions withinmemory buffer system 110. In one example, each step in an initializationor calibration sequence may be disabled or skipped by using aconfiguration bit.

In the present invention, the intermediate data transfers through memoryFIFO 130 and TX FIFO 132 may be optimized through optimizing a trainingsequence performed by calibration controller 150 for aligning clockphases for controlling clock boundaries at memory FIFO 130 and TX FIFO132, along with managing external feedback, and by optimizing theconfiguration of one or more components of memory FIFO 130, internaldata path 134, TX FIFO 132, and serializer 134. In particular, in atypical DDR memory interface system, the phase relation between a datastrobe that controls a clock of memory FIFO 130, a chip clock thatcontrols timing through internal data path 134 and TX FIFO 132, and anHSS clock that controls timing by serializer 134, during readoperations, may initially be unknown. In one example, calibrationcontroller 150 may utilize dedicated circuits of memory buffer system110 or circuits separate from memory buffer system 110 within system 100for performing system initialization and training to reduce the latencyin DDR read path 140 by aligning clock phases for controlling clockboundaries at FIFO 130 and TX FIFO 132. In one example, aligning clockphases may include one or more of inverting a clock or adjusting thephase of a clock.

In one example, calibration controller 150 may be implemented inconjunction with or separately from additional initialization andtraining of memory buffer system 110 performed to calibrate read andwrite controls of memory buffer system 110 by calibrating the voltagesand frequencies received by components for read and for writes withinmemory buffer system 110. For example, during read calibration,calibration controller 150 may optimize the gating of the arrivingstrobe by memory FIFO 130, align the data bits received by memory FIFO130 with the strobe, and center the strobe within the data eye. In oneexample, each data bit may have its own strobe, thereby enabling a phaserotator to align each DQ individually. In one example, during writecalibration, calibration controller 150 may include a fine writeleveling step for aligning the strobe with respect to a memory clock anda coarse write leveling step to adjust the strobe into a correct logicalwrite cycle. In addition, read calibration and write calibration mayinclude additional or alternate steps. In one example, memory buffersystem 110 may operate over a range of one or more conditions, includinga range of voltage settings, a range of frequency settings, a range oftiming settings, and a range of temperature refresh rates. Additionalconditions that may impact operation of memory buffer chip 110 includetiming, aging, and temperature.

In one example, in addition to calibration controller 150 performingclock inversions or phase alignments to minimize the latency incurredfrom intermediate data transfers through memory FIFO 130 and TX FIFO 132across multiple clock boundaries, system 100 may also implementadditional system optimization control to minimize read data latency.For example, system 100 may include controllers in memory buffer system110, HSS interface 112 and host 114 that minimize read data latency byminimizing the number of read data requests or read tag responsestransferred through system 100, such as by optimizing the scheduling ofread data requests by host 114 into more efficient memory data accesssequences that reduce cumulative read data latency. For example, memorybuffer system 110 may detect two or more memory data fetches from one ormore hosts targeting data within a common memory region and schedule theread data requests in an efficient memory data access sequence thatreduces overall read data latency.

In one example, while system 100 may also be configured and optimizedfor managing synchronous read data requests where read tag responses toread requests are delivered back to a host on a precisely determinedcycle, in the example, calibration controller 150 may optimize memorybuffer system 110 for minimizing read data latency from asynchronousread data protocol requests from host 114, by optimizing theintermediate data transfers through memory FIFO 130 and TX FIFO 132, toallow the read tag response to be potentially returned one or morecycles earlier than if not optimized.

FIG. 2 illustrates a block diagram of one example of a memory buffersystem configured in a distributed memory buffer topology with adedicated connection to an HSS interface.

In one example, a memory buffer system 210 may include multiple circuitelements, including, but not limited to, multiple DRAM devices andmemory buffer chips configured in a distributed memory buffer topology,with a dedicated connection to an HSS interface. In one example, memorybuffer system 210 includes multiple sets of DRAM devices illustrated bya DRAM set 212, DRAM set 214, DRAM set 216, and DRAM set 218, which maybe implemented in DRAM devices 108. In one example, each set of DRAMdevices illustrated may include multiple DRAM chips, such as the twoDRAM chips illustrated and more DRAM chips, including, but not limitedto, 8 more DRAM chips. In additional or alternate examples, memorybuffer system 210 may include additional or alternate sets of DRAM chipsand numbers of DRAM chips.

In one example, memory buffer system 210 may include one or more chipswhich may be implemented in memory buffer chip 106, including, but notlimited to, an address/command (AC) chip 230, for processing command,address, and control information and one or more data chips (DC)s, forprocessing data. In one example, memory buffer system 210 includes a DCset 220 of multiple DCs, such as DC 221, and a DC set 222 of multipleDCs, such as DC 223. In additional or alternate embodiments, memorybuffer system 210 may include additional AC chips and additional oralternate DC chips.

In one example, each of AC chip 230 and the DC chips in DC set 220 andDC set 222 have dedicated SERDES connections for interfacing with an HSSinterface to an HSS channel. In one example, AC chip 230 has a dedicatedHSS interface connection 240 to a processor address bus and a dedicatedHSS interface connection 241 back to the host for sending backresponses, error indicators, interrupts, and other types of responses.In addition, in one example, DC 221 has dedicated HSS interfaceconnections illustrated by inputs 232 and outputs 234 and DC 223 hasdedicated HSS interface connections illustrated by inputs 236 andoutputs 238. In one example, each of pair of inputs 232, inputs 236,outputs 234 and outputs 238 includes a connection to a “processor dataport 0” and a “processor data port 1”. In one example, the data outputfrom outputs 234 and outputs 238 may represent data signals, referred toas DQ.

In one example, AC chip 230 may have connections to each of the DRAMsthrough Register Clock Driver (RCD) devices, such as RCD 240 and an RCD242. For example, AC chip 230 may connect to each of the DRAM throughone of RCD 240 and RCD 242, either through a connection labeled “addr A”or “addr B”. In one example, each of RCD 240 and RCD 242 may representan address and control buffer that generate command sequences fordriving each of the DRAM. For example, AC 230 may connect to DRAM set212 through RCD 240 on “addr A”, AC 230 may connect to DRAM set 214through RCD 240 on “addr B”, AC 230 may connect to DRAM set 216 throughRCD 242 on “addr A”, and AC 230 may connect to DRAM set 218 through RCD242 on “addr B”. In one example, RCD 240 and RCD 242 may be implementedin memory buffer system 210 to handle electrical loading, depending onthe number of DRAMs present and the operational frequency of memorybuffer system 210. In additional or alternate embodiments, memory system210 may be implemented without RCD 240 and RCD 242. For example, wherememory buffer system 210 is contained in a DIMM, the signal lengths maybe short enough to allow AC 230 to communicate directly with the command(cmd)/address (addr), and control (cntrl) I/O of the DRAMs directly,rather than require the electrical drive capacity provided by passingthrough RCD 240 and RCD 242, such as an example including distributedbuffers mounted on a system planar and driving through a connector toIndustry Standard DIMMs.

In one example, each of the DC chips in DC set 220 and DC set 222 havesynchronous DRAM DDR connections to one or more DRAMs. For example, eachDC chip connects to a pair of DRAM on a “data port 0” and a pair of DRAMon a “data port 1”. In one example, DC 221 may have synchronous DDRconnections to a pair of DRAM in DRAM set 212 on a “data port 0”, DC 221may have synchronous DDR connections to a pair of DRAM in DRAM set 216on a “data port 1”, DC 223 may have synchronous DDR connections to apair of DRAM in DRAM set 214 on a “data port 0”, and DC 223 may havesynchronous DDR connections to a pair of DRAM in DRAM set 218 on a “dataport 1”.

In one example, each of AC chip 230 and the DC chips in DC set 220 andDC set 222 have dedicated SERDES connections for interfacing with an HSSinterface. In one example, AC chip 230 has a dedicated HSS interfaceconnection 240 to a processor address bus. In addition, in one example,DC 221 has dedicated HSS interface connections illustrated by inputs 232and outputs 234 and DC 223 has dedicated HSS interface connectionsillustrated by inputs 236 and outputs 238. In one example, each of pairof inputs 232, inputs 236, outputs 234 and outputs 238 includes aconnection to a “processor data port 0” and a “processor data port 1”.In particular, in the example, outputs 234 and outputs 238 may representdedicated DDR read paths to one or more HSS interfaces.

In one example, AC chip 230 is connected to each of the DC chips in DCset 220 and DC set 222 for sending data buffer commands through abroadcast communication bus (BCOM). For example, AC 230 broadcastsinformation to DC 221, and other DC chips in DC set 220 on “BCOM A” andbroadcasts information to DC 223, and other DC chips in DC set 222 on“BCOM B”.

In one example, memory buffer system 210 may include one or more clocktopology configurations. For example, AC 230 may generate a broadcastclock (BCLK) signal, from a BCLK 231. In one example, each of RCD 240and RCD 242, or AC 230 if RCDs are not implemented, may broadcastinformation to all the DRAM within memory buffer system 210 using acommon clock and communication path, with the common clock driven byBCLK 231. In one example, the common clock driven by BCLK 231 may drivea DC phase locked loop (PLL), which drives a DQS signal for data outputon outputs 234 and outputs 238, and the PLL may drive an internal DCchip clock.

FIG. 3 illustrates a block diagram of one example of a memory buffersystem configured in a unified memory buffer topology with a dedicatedconnection to an HSS interface.

In one example, a memory buffer system 310 may include multiple circuitelements, including, but not limited to, multiple DRAM devices and amemory buffer chip configured in a unified memory buffer topology. Inone example, the unified memory system topology represents an OpenCAPIarchitecture that allows any microprocessor attached to advancedmemories accessible via read/write or user-level DMA semantics.

In one example, memory buffer system 310 is illustrated with multiplesets of DRAM devices illustrated by a DRAM set 322 and DRAM set 324,which may be implemented in DRAM devices 108, a unified memory buffer(UB) 320, which may be implemented in memory buffer chip 106, and avoltage regulator (reg) 330 for driving the power grid of memory buffersystem 310. In one example, each set of DRAM devices illustrated mayinclude multiple DRAM chips. For example, DRAM set 322 illustrates 10DRAM chips connected on a 40 bit data bus to UB 320 and DRAM set 324illustrates 8 DRAM chips connected on a 32 bit data bus to UB 320. Inone example, UB 320 may control an address bus, illustrated as “addressbus A” to the DRAM chips in DRAM set 322 and as “address bus A1” to theDRAM chips in DRAM set 324.

In one example, UB 320 may include a dedicated DDR read path forconnecting memory buffer system 310 to a host through an HSS interface,illustrated by connections 340. In additional or alternate embodiments,UB 320 may include additional or alternate components and connections.

FIG. 4 illustrates a block diagram of one example of a generalimplementation of a DDR read path of an HSS interface within a memorybuffer system.

In one example, a DDR read path 400 may require data to cross multipleclock boundaries, illustrated by clock boundary 120 and clock boundary122, where each clock boundary is illustrated between a different clockdomain. In one example, DDR read path 400 may include additional oralternate clock boundaries and additional or alternate domains.

In one example, a first domain 410 may represent memory FIFO 130,providing an interface for latching read request response data signal“DQ” from a DRAM chip to a read FIFO 412 and a read FIFO 414. In oneexample, read FIFO 412 may represent a stack of FIFOs or FIFO bufferasynchronously receiving data from a different port of a distributedbuffer or unified buffer memory topology. In one example, each of readFIFO 412 and read FIFO 414 include a RDCLK signal that controls a clockfor buffering data from data signal “DQ”. In one example, the RDCLK isdriven by a data strobe signal “DQS”. In one example, DQS is a datastrobe signal generated in a DDR DRAM interface, such as is illustratedin FIG. 2 and FIG. 3. In one example, the DQS may be generated by aninternal clock of each DC and a phase locked loop (PLL) 424 inside theDC sets used to generate and align the DQS to outgoing data DQ. In oneexample, there may be a skew between the DQS signals arriving at each ofthe read FIFOs. In one example, a phase adjust 416 and phase adjust 418may be implemented to phase align the DQS signals generated by each DC,as a way of centering each DQS edge to capture read data within a datavalid window. In one example, DQS may be phase aligned with incoming DQdata, in combination with buffering the DQ data through two buffers, sothat each of read FIFO 412 and read FIFO 414 may clock in and bufferdata on a positive and negative edge of the DQS signal.

In one example, each of the DQS signals may have rising and fallingedges during different clock cycles or during different phases of a sameclock cycle, leading to read FIFO 412 and read FIFO 414 clocking DQ atdifferent times. A DDR read, in DDR protocol, may include reading outfrom read FIFO 412 and read FIFO 414 in parallel. In the example, theskew between DQS signals may cause skew between the data strobes of theDQS inputs to each of read FIFO 412 and read FIFO 414, such that thelatest arriving strobe from each DQS signal controls the end of eachlane read and the latest arriving strobe may arrive one or more clockcycles after the earlier data strobe.

In one example, a second domain 420, may represent internal data path131 and TX FIFO 132. In one example, second domain 420 includes a readpointer (RD PTR) that controls reading data out of read FIFO 412 andread FIFO 414, across clock boundary 120 through a selection of a 4:1multiplexer(mux) 440 and flip flop 444 or a 4:1 mux 442 and flip flop446, followed by a 2:1 mux 448, before buffering in TX FIFO 426.

In one example, a clock signal controlling data flow through domain 420is controlled by a chip clock 422, which distributes a DC chip clock. Inone example, chip clock 422 receives an output from PLL 424, divided bya divider 423 into a single output running four times slower than theinput from PLL 424. In one example, PLL 424 may implement a voltagecontrolled oscillator to adjust a timing relationship between an inputclock signal, such as a buffered BCLK signal, and output data. In oneexample, divider 423 may divide an 8 Ghz signal into a slower 2 Ghzsignals, to accommodate for phase misalignments between BCLK, such asthe internal clock of AC 230, and the latest arriving strobe from DQS.In particular, by slowing down the signals, more time is allowed throughdomain 420 to accommodate for phase misalignments.

In one example, an MC 428 in domain 420 receives the clock signal fromchip clock 422. MC 428 may control a read pointer (RD PTR) for read dataout of read FIFO 412 into mux 440 and for read data out of read FIFO 414into mux 442. In one example, the RD PTR may control which beats of datato read out of read FIFO 412 and read FIFO 414. In one example, eachread FIFO may hold 2 beats of data and each DRAM may return 8 beats ofdata, so the RD PTR may cycle through 4 entries to obtain a cohesiveline of data. In addition, MC 428 may control a clock signal to clockthe output from mux 440 into flip flop 444 and the output from mux 442into flip flop 446. In addition, MX 428 may control a signal to selectdata from the 2:1 mux 448 for switching between ports for buffering inTX FIFO 426 for multi-port configurations. In one example, TX FIFO 426receives a clock signal from chip clock 422 for controlling buffering ofdata from mux 448. In addition, a run_count strobe 450 and unloadpointer 452 receive a clock signal for controlling each flip flop fromchip clock 422. In particular, by adding clocked elements, such as flipflop 444 and flip flop 446, additional clock cycles of latency are addedto accommodate for phase misalignments.

In one example, a third domain 430 may represent serializer 134. In oneexample, third domain 430 includes an unload pointer that controlsreading data out of TX FIFO 426, across clock boundary 122 through aselection of a 4:1 mux 436, an 8:2 serializer 432, and a frequencyamplifier 438. In one example, in third domain 430, a 4:1 divider 434divides a signal from 8 Ghz to 2 Ghz. In one example, unload pointer 452is a counter clocked by the 2 Ghz signal from 4:1 divider 434 in domain430. In one example, a run_count strobe 450 may select between thedifferent phases of divided clock signal of 4:1 divider 434, forcontrolling the phase of the unload pointer 452 to mux 436. Mux 436outputs data to an 8:2 serializer 432, which converts parallel data intoserialized data and outputs the serialized data to amplifier 438, whichincreases the signal frequency to the frequency output by PLL 424. Inthe example, the path from 4:1 divider 434 to unload pointer 452,through mux 426 and back to serializer 432 is synchronized.

In the example, each of read FIFO 412 and read FIFO 414 at clockboundary 120 and TX FIFO 126 at block boundary 122 are positioned toabsorb phase misalignment between BCLK and DQS. In particular, onelimitation of DDR read path 400 is that when operating, there will beskew between BCLK and DQS signals. In addition, even though the unloadpointer in domain 430 and the read clock into TX FIFO 426 in domain 420are technically driven by PLL 424, the same PLL, because of differencesin physical distribution in a system, the domains have the same clocksource, but with different alignments of the clock source.

This misalignment, as illustrated in FIG. 4, is compensated for bysufficiently separating the unload pointer in domain 430 from the RDpointer in domain 420 by a certain number of clock cycles to ensure datastability before reading the data into a serializer. Adding the clockcycles to separate the pointers increases latency. In addition, thismisalignment, as illustrated in FIG. 4, is compensated for by divider434 of domain 430 dividing the 8 Hz signal, and taking 1 of 4 possiblephases of an 8 Hz signal during chip initialization, as set by run count450, which increases the chances of non-optimal alignment across the DCclock boundaries. As illustrated in FIG. 4, the SERDES FIFO may absorbthis misalignment by way of increasing the unload time with respect tothe loading of the TX FIFO 426, thereby further increasing the datareturn latency.

In the present invention, calibration controller 150 may optimize theDDR read path 400 by aligning the phases between the DQS strobe indomain 410, chip clock 422 in domain 420, and the HSS clock driving theunload pointer in domain 430. In one example, calibration controller 150may align the chip clock 422 to the latest arriving DQS strobe and thenalign the HSS clock driving the unload pointer in domain 430 to chipclock 422, to minimize the latency on DDR read path 400.

In particular, calibration controller 150 may initially start bystarting AC clocks, such as BCLK 231 of AC 230, sending the BCLK signalto each of the DC chips of DC set 220 and DC set 222, and initializing aBCOM connection between AC 230 and each of the DC chips, includinginitializing each of the DC chips. Next, calibration controller 150 maytrain an AC HSS link, which may also be referred to as the downstreampath, skipping the DC HSS link training, which may also be referred toas the upstream path, by setting a bit not to perform the DC HSStraining yet because at this point, training the link between the DCchips and the host would likely result in non-optimized clock crossings.In one example, training an AC HSS link may include setting run_count450, which selects a phase of 4:1 divider 434 to output for controllingthe unload pointer. Thereafter, calibration controller 150 may performmemory interface initialization, including read and write calibration,using the AC HSS link and BCOM link. In one example, read calibrationmay include identifying a latest arriving strobe when centering DQ andDQS signals into read FIFO 412 and read FIFO 414.

In one example, calibration controller 150 may determine whetherexternal feedback control is required. For example, in a distributedbuffer memory topology illustrated in FIG. 2, external feedback controlmay be required. In a unified buffer memory topology illustrated in FIG.3, external feedback control may not be required. In one example, a PLLfeedback path illustrated in FIG. 7, may be added to PLL 424 toaccommodate for external feedback control requirements in FIG. 4, inresponse to calibration by calibration controller 150, depending on thememory buffer topology. In one example, external feedback may berequired in a distributed memory buffer topology to keep the relativephase of PLL 424 across multiple DC chips, to keep the chips fromdrifting with respect to one another, but PLL 424 locks the phase afterinitialization, so a PLL feedback path is required to adjust the phaseafter initialization for aligning chip clock 422 with a data strobe.

In one example, if external feedback control is not required,calibration controller 150 may adjust a phase of chip clock 422, alsoreferred to as a DC chip clock, to align to the latest strobe and mayadjust DC chip write clock phase rotators. In particular, adjusting aphase of chip clock 422 to align to the latest strobe minimizes theamount of time the read data must remain in read FIFO 412 and read FIFO414 before it can be unloaded and transmitted to TX FIFO 426. However,the DC chip clock adjustment may upset the prior relationships that wereestablished during memory interface calibration, thereby obviatingseveral results. To compensate for the adjustments, the AC chip memorywrite clock and command/address/control phase rotators may be adjustedby an amount corresponding to the applied DC chip clock phase shift. Inthe example, the DC clocks may be shifted with any desired granularitydepending on the implementation complexity. For example, delay lines maybe used to provide four phases separated by 90 degrees, or phaserotators could be used to achieve a larger number of phases.

In one example, if external feedback is required, prior to adjusting thephase of chip clock 422 to align to the latest strobe and adjusting DCchip write clock phase rotators, calibration controller 150 may firstimplement a PLL feedback path with equal delay to adjust the phase ofchip clock 422.

In one example, to determine the latest strobe, one or more of hardwareand software may be implemented. In one example, read data path 400 mayinclude additional hardware circuits for monitoring the DQS signals toidentify the latest arriving strobe. In another example, software orfirmware may capture information about each DQS strobe and capture thedelay information, analyze the captured information, and determine whichDQS is the latest strobe.

In one example, following the adjustment of the phase of chip clock 422,calibration controller 150 may perform the DC HSS training step usingnewly adjusted chip clock 422 to tune the HSS clock to launch the unloadpointer by tuning a phase of 4:1 divider 434. In one example,calibration controller 150 may calibrate the phase of 4:1 divider 434,for controlling the HSS clock, using a current phase of run_count strobe450, as driven by the recently adjusted phase of chip clock 422. Thecalibrated phase of 4:1 divider 434 drives the phase of unload pointer452, triggering the unload pointer to unload mux 436. By calibrating thephase of 4:1 divider 434 using a current phase of run_count strobe 450,the phase of chip clock 422 driving data to be written into TX FIFO 426is calibrated with respect to the phase of unload pointer 452 fortriggering the unload pointer to unload data from TX FIFO 426, tominimize the time from when the data are written into TX FIFO 426 towhen the data are unloaded from mux 436 and transmitted to the host.

FIG. 5 is a timing diagram illustrating one example of an ideal clockand strobe alignment compared with a run time clock and strobemisalignment to be minimized by a calibration controller to minimizeread data latency between a memory buffer system and a host, across anHSS interface.

In one example, a timing diagram 510 illustrates an example of an idealclock and strobe alignment in FIG. 4, in an example including an HSSsignal, which may represent the signal received by 4:1 divider 434 indomain 430, a DQS signal, such as the DQS signal inputs to read FIFO 412and read FIFO 414, and a BCLK signal. In a first example, the rising andfalling edges of HSS signal 512, DQS signal 514, and BCLK 516 arealigned in an ideal clock and strobe alignment.

In one example, a timing diagram 520 illustrates an example of the clockand strobe misalignment that may occur at runtime in FIG. 4 because theincoming DQS signal to one read FIFO for clocking a nibble or byte of DQdata may be skewed with respect to the incoming DQS signal to anotherread FIFO for clocking a nibble or byte of DQ data. In one example, HSSsignal 522 is illustrated as a signal consistent with HSS signal 512 intiming diagram 510 and BCLK 529 is illustrated as a clock signalconsistent with BCLK 516, however, in a run time environment, the DQSsignals into read FIFO 412 and read FIFO 414 are not aligned. Forexample, DQS0 signal 524 may control a RDCLK into read FIFO 412 and DQS1signal 526 may control a RDCLK into read FIFO 414. In the example, intiming diagram 520, the strobe from DQS0 524 and the strobe from DQS1526 are not aligned with one another, and are also not aligned withrising or falling edges of the clock signals of HSS signal 522 and BCLK528. In real time operation, the strobes from DQS0 524 and DQS1 526 maybe skewed across multiple nibbles or bytes, introducing additional clockcycles of latency into the read data path, requiring additional cyclesof latency to be added between clock boundaries to accommodate for datastrobes to trigger clocking of data at different clock times, and towait for a latest arriving strobe for a lane of data read out in a DDRprotocol, to deskew and align data such that a cohesive cache line canbe delivered.

In one example, a timing diagram 530 illustrates an example ofadditional clock and strobe misalignments that may occur at runtime inFIG. 4 if additional memory buffer chips are added in a multi-portstructure, with multiple DC chips. In one example, as additional memoryDC chips are added in a multi-port system, in addition to the DQS skewintroduced in timing diagram 520, additional skew is introduced becausethe incoming BCLK signal on one DC chip may be skewed with respect tothe same BCLK signal arriving on a different DC chip. Since host 114needs to obtain data from all the DC chips of the memory buffer chipsimultaneously, the skew introduced by the BCLK signal arriving on oneDC chip at one time and another DC chip at another time increase theimpact of skew on the latency of a read data return path of the memorybuffer chip. In one example, HSS signal 532 is illustrated as a signalconsistent with HSS signal 512 and HSS signal 522. DQS0 534 and DQS1 536reflect the skewed signals of DQS0 524 and DQS1 526. In addition, intiming diagram 530, multiple BCLK signals are illustrated. A BCLK1 538signal represent a BCLK signal arriving at a first DC chip in a firstport and BCLK2 540 signal represent the BCLK signal arriving at a secondDC chip in a second port. In one example, BCLK 1 538 and BCLK2 540 areskewed, introducing additional clock cycles of latency into the readdata path, in addition to the additional clock cycles of latencyintroduced by the DQS skew, requiring additional cycles of latency to beadded, to deskew and align data from multiple ports, such that acohesive cache line can be delivered.

FIG. 6 illustrates a block diagram of one example of a DDR read path ofan HSS interface optimized for multiple memory buffer chips in amulti-port memory buffer system.

In one example, a first domain 610 of a DDR read path 600 may representa memory FIFO 130, supporting multiple memory buffer chips, in amulti-port memory buffer system. In one example, first domain includesmultiple DDR read FIFOs, for each DDR port. For example, a DDR port 00612 includes a DDR read FIFO 614, a DDR read FIFO 616, a DDR read FIFO618, and a DDR read FIFO 620. In one example, each of the DDR read FIFOsof DDR port 00 612 are specified for one of a selection of 4 HSSinterface lanes in each memory buffer chip, numbered 0-3, with a 2×8buffer, for holding 16 bits of data. For example, DDR read FIFO 614holds lane 0 data, DDR read FIFO 616 holds lane 1 data, DDR read FIFO618 holds lane 2 data, and DDR read FIFO 620 holds lane 3 data. In oneexample, each “lane” may represent a nibble or byte of data, wheredepending on the type of DRAM device, such as an X4 or X8 device type, aseparate read FIFO may be implemented to handle each nibble or byte ofdata supported in parallel on the memory buffer chip. In one example,each of the DDR read FIFOs for each of DDR port 00 620, DDR port 01 622,DDR port 10 624, and DDR port 11 626, may receive a DQ signal as a datainput, with buffering, as illustrated with respect to the DQ signal inFIG. 4, and may receive a DQS signal as a clock input, includingbuffering and phase adjustment, as illustrated with respect to the DQSsignal in FIG. 4.

In one example, depending on the skew between DQS signals of each of theDDR read FIFOs of each port, data may be clocked into the DDR read FIFOsfor each lane of each port at different times. In one example, datacross from the DDR read FIFOs across clock boundary 120 and are latchedand multiplexed by each of the DDR port mux based on a DDR port select632, from the BCOM signal. In the example, the DDR port mux include amux 634 for lane 0 data from each port, a mux 636 for lane 1 data fromeach port, a mux 636 for lane 2 data from each port, and a mux 640 forlane 3 data from each port. In one example, a DDR port select 832,provides a selection of inputs by port, for each lane, to mux 634, mux636, mux 638, and mux 640, based on the BCOM signal for controlling thedata commands by port. In one example, DDR port select 632 may determinewhich beats of data to read out of each DDR read FIFO, where each DDRread FIFO may hold 2 beats of data, where each DRAM may return 8 beatsof data, such that DDR port selection 632 may cycle through 4 FIFOentries, or slots, to obtain a cohesive line of data.

In one example, returning to domain 630, the selected lane outputs fromthe DDR port mux is received as input to DDR read cycle mux and islatched by each of the DDR read cycle mux based on a DDR read cycleselect 642, from the BCOM signal. In one example, the DDR read cycle muxmay include a mux 644 for multiplexing lane 0 data [0:15] to data [0:1],a mux 646 for multiplexing lane 1 data [0:15] to data [2:3], a mux 648for multiplexing lane 2 data [0:15] to data [4:5], and a mux 650 formultiplexing lane 3 data [0:15] to data [6:7]. In the present invention,based on an optimization by calibration controller 150, DDR port select632 may read out 1-2 bytes from each FIFO, at the same time, perfectlyaligned across all ports. In one example, the DDR read cycle select 642may select between ports in a multi-port configuration and may bepositioned based on one clock cycle of uncertainty in a local clockcycle selection.

In particular, in the example, chip clock 656 may distribute a PCLKsignal divided by a 4:1 divider 690. In one example, chip clock 656 maydrive a clock signal to all of the logic associated with unloading datafrom the DDR read FIFOs, such as DDR port select 632 and DDR read cycleselect 642. In one example, in the present invention, to minimizelatency on DDR read path 600, calibration controller 150 performs aphase alignment of chip clock 656 with respect to a latest arriving DQSsignal at the DDR read FIFOs, also referred to as a latest arrivingstrobe.

In addition, chip clock 656 may drive a clock signal to the logic forcontrolling writing data into TX FIFO 664. In one example, run_count 666may represent a strobe that is derived from chip clock 656. An unloadpointer 673 is a function of run_count 666. As will be furtherdescribed, calibration controller 150 performs a phase alignment tooptimize the phase of assertion of an unload pointer 673 to controlwriting data into TX FIFO 664.

In one example, the data multiplexed from the DDR read cycle mux may beoutput as data [0:7] 652 and pass through eight data wires, routed atchip level and captured synchronously. In one example, a mux 653receives the data [0:7] 652 as one input and receives an in-band accesspath [0:7] 654 as another input. In one example, an in-band select 660is a pointer into mux 653 to select between data [0:7] 652 and in-bandaccess path [0:7] 654. In one example, in-band access path [0:7] 654 maybe selected for enabling the host firmware (FW) to access informationinside the memory buffer chip through DDR read path 600, in-band,through the same channels as the ports. The host FW may use in-bandaccess path [0:7] 654 for sending read and write operations that targetinternal FW registers, as opposed to memory. In one example, FWoperations may be infrequent and may not be critical for performance,such as FW operations during memory interface calibration, so FWoperations may be routed through a different clock distribution thatruns at a slower frequency with asynchronous boundaries, to save power,regardless of which speed memory is configured in the system. In oneexample, in-band access path [0:7] 654 may not be affected by updatesperformed to the phases of chip clock 656 by calibration controller 150and if the new phases optimized by calibration controller 150 result inadding more latency to FW operations, given that FW operations areinfrequent and not performance critical, the additional latency to FWoperations does not significantly impact the overall performance of thememory buffer system. In one example, data [0:7] 652 may also be outputto cyclic redundancy check (CRC) check 658. In one example, a CRC checkmay represent an error detecting code used to detect accidently changesin data.

In one example, the output of mux 653 is logically OR′d with an outputfrom a pseudo-random binary sequence (PRBS) scrambler 662 into a TX FIFO664. In one example, PRBS scrambler 662 may transform the input datastream for ensuring accurate recovery on a receiver, such as the host,where the host may implement a receiving LFSR to descramble thescrambled input data stream based on a sync-word between PRBS scrambler662 and a receiving LFSR.

In one example, TX FIFO 664 may include a 32 bit buffer and output FIFOdata [0:31] 668. In one example, FIFO data [0:31] 668 crosses clockboundary 122 from domain 630 to a domain 670. In one example, a mux 672latches a selection of FIFO data [0:31] 668 based on an unload pointerinput and passes a selection of 8 bits of data through a voltage levelsLVL TRANS buffer, as data [0:7] 678, based on an unload pointer signal.In one example, a 4:1 divider 684 may divide a clock signal receivedfrom PLL 688, to drive the unload pointer to mux 672. In one example,the divided clock signal passes through a voltage level LVL TRANS bufferto an unload pointer count.

In one example, an 8:2 serializer 674 receives data [0:7] 678 andserializes the data into 1 bit serial data [0:1] 680. In one example, anamplifier 682 amplifies the frequency of data [0:1] 680 by the frequencyof the signals output from a PLL 688.

In one example, while PLL 688 may drive both an HSS clock, illustratedby C2_CLK_T and C2_CLK_C, and a DC chip clock 656, illustrated by PCLK,the HSS clock and DC chip clock do not originally have a known phaserelationship. In particular, in the example, referring back to FIG. 2,calibration controller 150 may first initialize AC 230 to establishinternal clocks and a communication path to the DC chips in DC set 220and DC set 222. In one example, initializing AC 230 may include startingAC clocks, including BCLK 231, sending the BCLK signal to each of theDCs in DC set 220 and DC set 222 and establishing the BCOM signalbetween AC 230 and each of the DCs in DC set 220 and DC set 222. In oneexample, part of the AC chip initialization performed by calibrationcontroller 150 may include training the HSS interface and establishinginternal AC clock phases. In addition, calibration controller 150 mayinitially initialize the DCs in DC set 220 and DC set 222 to establishan access path needed for memory interface initialization and training.At this point, the HSS interface between the DCs and the host could alsobe trained, however, doing so would likely result in non-optimized clockcrossings which contribute to additional latency in DDR read path 600.

In the example, calibration controller 150 optimizes phase alignmentsbetween memory buffer chips and host through DDR read path 600 byforegoing the initial HSS interface training of DDR read path 600between the DC chips and the host prior to memory interfaceinitialization and training. Calibration controller 150 uses the ACtrained HSS interface and the BCOM signal for memory interface trainingand training for both the AC and DC chips. In the example, the memoryinterface training and training for both the AC and DC chips mayinclude, but is not limited to, write leveling, strobe alignment,internal DDR PHY clock alignment at chip clock 656, and read leveling.As a result of the memory interface training and training for both theAC and DC chips, a fixed relationship is established between theincoming data strobe DQS on each DC chip, the memory clock emanatingfrom the AC, and the internal AC clock.

In particular, in the example illustrated in FIG. 2, since each DC chipinternal clock is sourced from the AC as BCLK, under perfect operatingconditions, the DC internal clock for generating DQS would be phasealigned to the AC internal clock. In reality, the presence of skewbetween the AC chip and the multitude of DC chips, the transmissionmedium, and the presence of the PLL within the DC chip all conspire toshift the relationship of DC clocks with respect to AC clocks. As aresult, the chip clock 656 in DDR read path 600 will likely not bealigned with the incoming data strobe DQS, and its corresponding DDR PHYread clock.

In the present invention, in a similar manner as described withreference to FIG. 4, calibration controller 150 is optimized to minimizethe latency through the DDR read FIFOs on multiple DDR ports and TX FIFO664 by optimizing various clock phases. Calibration controller 150 firstimplements a circuit to assess the arrival times of the DC data strobesfrom the DQS signal, upon completion of the memory interface training.Next, calibration controller 150 determines whether external feedback isrequired. If the memory topology of the memory buffer chip is adistributed memory buffer topology, as described in FIG. 2, externalfeedback is required to manage for skew among the DC chips for PLL 688.If the memory topology of the memory buffer chips is a unified memorybuffer topology, as described in FIG. 3, then external feedback is notlikely required.

In one example, if external feedback control is not required, aspreviously described with reference to FIG. 4, calibration controller150 may adjust a phase of chip clock 658 to align to the latest strobeand may adjust DC chip write clock phase rotators. In particular,adjusting a phase of chip clock 658 to align to the latest strobeminimizes the amount of time the read data must remain in the read FIFOsbefore being unloaded and transmitted to TX FIFO 664. However, the DCchip clock adjustment may upset the prior relationships that wereestablished during memory interface calibration, thereby obviatingseveral results. To compensate for the adjustments, the AC chip memorywrite clock and command/address/control phase rotators may be adjustedby an amount corresponding to the applied DC chip clock phase shift. Inthe example, the DC clocks may be shifted with any desired granularitydepending on the implementation complexity. For example, delay lines maybe used to provide four phases separated by 90 degrees, or phaserotators could be used to achieve a larger number of phases.

In one example, if external feedback is required, calibration controller150 may apply a 180 degree phase shift, or full inversion, of thealternate edge of DC chip clock 656, by selecting a simple selectableinversion. Allowing the full inversion may also require additionalcircuit logic to permit unloading the DDR FIFO either through rising orfalling edge capture latches. In the example, the inversion option issimpler and less costly to implement, however, may also provide amaximum latency reduction of half a clock cycle.

In one example, to determine the latest strobe, one or more of hardwareand software may be implemented. In one example, read data path 400 mayinclude additional hardware circuits for monitoring the DQS signals toidentify the latest arriving strobe. In another example, software orfirmware may capture information about each DQS strobe and capture thedelay information, analyze the captured information, and determine whichDQS is the latest strobe.

In the example, calibration controller 150 further minimizes the latencythrough FIFO 664, across clock domain 120 by using the previouslyadjusted phase of DC chip clock 656. In one example, calibrationcontroller 150 may next run an HSS training set that compares thecurrent TX launch clock phase driven to run_count strobe 666, as drivenby the recently adjusted phase of DC chip clock 656. In particular, toaccount for a quarter cycle of clock uncertainty in the clock cycleselection for the unload pointer, the phase of 4:1 divider 684 may becalibrated during training using run_count 666, as a strobe from DC chipclock 656. In particular, calibration controller 150 may compare acurrent TX launch clock phase to run_count 666, and set a phase of 4:1divider 684 of unload pointer 673, to optimize the phase of unloadpointer 673 and to minimize the amount of time from when the data arewritten into TX FIFO 664 by unload pointer 673 to when data are unloadedand transmitted to the host.

In one example, a selection of components 694 of DDR read path 600 mayrepresent a first transmit differential memory interface (DMI) and aselection of components 696 of DDR read path 600 may represent a secondtransmit DMI. In one example, a DMI may represent a physical layerinterface that enables the transport of memory command, address, anddata encapsulated in frames through high speed Serdes links to and fromthe host CPU.

In one example, in view of the clock and strobe misalignment from DQSskew that is introduced in real time conditions in timing diagram 520and in view of the additional clock and strobe misalignment from DQSskew and BCLK skew that is introduced in a multiport system withmultiple memory buffer chips in timing diagram 530, by implementingcalibration controller 150 of the present invention to optimize phasealignment also optimizing the components of the memory interface, theinternal data path, and the HSS interface to enable calibrationcontroller 150 to further optimize phase alignment, calibrationcontroller 150 may minimize the delays introduced in the read path fromdata buffers and crossing multiple clock boundaries between memorybuffer chips and a host through an HSS interface.

FIG. 7 illustrates one example of a block diagram of external feedbackcontrol of a PLL.

In one example, external feedback control of a PLL 700 illustrates anexample of a PLL 720 implemented with a feedback path 720, in the eventthat a DC chip clock requires an additional external feedback mechanismfor a PLL, to compensate for the adjustments to prior relationshipsestablished during memory interface calibration.

In one example, a PLL 710 may receive a clock signal of REF CLK 714,which is driven by the BCLK. In one example, PLL 710 may include afeedback pin (FDBK) 712. In one example, PLL 710 include two outputs,illustrated as an “out_A” 716 and an “out_B” 718.

In one example, regardless of whether an external feedback path existsor not, “out_A” 716 may be used as the high speed (8 Ghz) output fromPLL 424 to divider 4:1 434 to specifically driver 8:2 serializer 432 orhigh speed output “C2_CLK_T” from PLL 688 input to divider 4:1 684 tospecifically driver 8:2 serializer 674.

In one example, in the present invention, as to “out_B” 718, calibrationcontroller 150 may run memory interface training and determine a latestarriving strobe. PLL 710 may perform clock distribution with finegranular phase adjustment on DDR PHY 732 to align an internal clock tree730 connected to “out_B” 718 to the latest arriving strobe. In oneexample, DDR PHY 732, may represent the clock phase at chip clock 422and chip clock 656. In the example, since internal clock tree 730 is notconnected to FDBK 712, the alignment of internal clock tree 730 willremain intact. Next, during DC HSS training, calibration controller 150may use the new core clock alignment of internal clock tree 730 to matchup to the TX clock of HSS PHY 734. In one example, HSS PHY 734 mayrepresent the clock phase arriving at 4:1 divider 434 and the clockphase on C2_CLK_C 688, arriving at 4:1 divider 684. The result is aphase aligned path from the latest arriving strobe in the first domain,to the FIFO read pointer for reading from the read FIFO into the seconddomain, to the unload pointer for reading from the TX FIFO into thethird domain.

In the example, PLL 710 may include a feedback path 720 for handling anyfuture process, voltage, and temperature (PVT) variation that may occurthat would endanger calibration results. In the example, in the event ofPVT variation, in one example, calibration controller 150 may trigger anexternal feedback path 720, from “out_A” 716, to null out the PVTvariation. In one example, external feedback path 720 may include afeedback path with matching delay (D) for tracking PVT variation.

FIG. 8 illustrates a block diagram of one example of a computer systemin which one embodiment of the invention may be implemented. The presentinvention may be performed in a variety of systems and combinations ofsystems, made up of functional components, such as the functionalcomponents described with reference to a computer system 800 and may becommunicatively connected to a network, such as network 802.

Computer system 800 includes a bus 822 or other communication device forcommunicating information within computer system 800, and at least onehardware processing device, such as processor 812, coupled to bus 822for processing information. Bus 822 preferably includes low-latency andhigher latency paths that are connected by bridges and adapters andcontrolled within computer system 800 by multiple bus controllers. Whenimplemented as a server or node, computer system 800 may includemultiple processors designed to improve network servicing power.

Processor 812 may be at least one general-purpose processor that, duringnormal operation, processes data under the control of software 850,which may include at least one of application software, an operatingsystem, middleware, and other code and computer executable programsaccessible from a dynamic storage device such as random access memory(RAM) 814, a static storage device such as Read Only Memory (ROM) 816, adata storage device, such as mass storage device 818, or other datastorage medium. Software 850 may include, but is not limited to, code,applications, protocols, interfaces, and processes for controlling oneor more systems within a network including, but not limited to, anadapter, a switch, a server, a cluster system, and a grid environment.

Computer system 800 may communicate with a remote computer, such asserver 840, or a remote client. In one example, server 840 may beconnected to computer system 800 through any type of network, such asnetwork 802, through a communication interface, such as networkinterface 832, or over a network link that may be connected, forexample, to network 802.

In the example, multiple systems within a network environment may becommunicatively connected via network 802, which is the medium used toprovide communications links between various devices and computersystems communicatively connected. Network 802 may include permanentconnections such as wire or fiber optics cables and temporaryconnections made through telephone connections and wireless transmissionconnections, for example, and may include routers, switches, gatewaysand other hardware to enable a communication channel between the systemsconnected via network 802. Network 802 may represent one or more ofpacket-switching based networks, telephony based networks, broadcasttelevision networks, local area and wire area networks, public networks,and restricted networks.

Network 802 and the systems communicatively connected to computer 800via network 802 may implement one or more layers of one or more types ofnetwork protocol stacks which may include one or more of a physicallayer, a link layer, a network layer, a transport layer, a presentationlayer, and an application layer. For example, network 802 may implementone or more of the Transmission Control Protocol/Internet Protocol(TCP/IP) protocol stack or an Open Systems Interconnection (OSI)protocol stack. In addition, for example, network 802 may represent theworldwide collection of networks and gateways that use the TCP/IP suiteof protocols to communicate with one another. Network 802 may implementa secure HTTP protocol layer or other security protocol for securingcommunications between systems.

In the example, network interface 832 includes an adapter 834 forconnecting computer system 800 to network 802 through a link and forcommunicatively connecting computer system 800 to server 840 or othercomputing systems via network 802. Although not depicted, networkinterface 832 may include additional software, such as device drivers,additional hardware and other controllers that enable communication.When implemented as a server, computer system 800 may include multiplecommunication interfaces accessible via multiple peripheral componentinterconnect (PCI) bus bridges connected to an input/output controller,for example. In this manner, computer system 800 allows connections tomultiple clients via multiple separate ports and each port may alsosupport multiple connections to multiple clients.

In one embodiment, the operations performed by processor 812 may controlthe operations of flowchart of FIG. 9 and other operations describedherein. Operations performed by processor 812 may be requested bysoftware 850 or other code or the steps of one embodiment of theinvention might be performed by specific hardware components thatcontain hardwired logic for performing the steps, or by any combinationof programmed computer components and custom hardware components. In oneembodiment, one or more components of computer system 800, or othercomponents, which may be integrated into one or more components ofcomputer system 800, may contain hardwired logic for performing theoperations of flowcharts in FIG. 9.

In addition, computer system 800 may include multiple peripheralcomponents that facilitate input and output. These peripheral componentsare connected to multiple controllers, adapters, and expansion slots,such as input/output (I/O) interface 826, coupled to one of the multiplelevels of bus 822. For example, input device 824 may include, forexample, a microphone, a video capture device, an image scanning system,a keyboard, a mouse, or other input peripheral device, communicativelyenabled on bus 822 via I/O interface 826 controlling inputs. Inaddition, for example, output device 820 communicatively enabled on bus822 via I/O interface 826 for controlling outputs may include, forexample, one or more graphical display devices, audio speakers, andtactile detectable output interfaces, but may also include other outputinterfaces. In alternate embodiments of the present invention,additional or alternate input and output peripheral components may beadded.

With respect to FIG. 8, the present invention may be a system, a method,and/or a computer program product. The computer program product mayinclude a computer readable storage medium (or media) having computerreadable program instructions thereon for causing a processor to carryout aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Those of ordinary skill in the art will appreciate that the hardwaredepicted in FIG. 8 may vary. Furthermore, those of ordinary skill in theart will appreciate that the depicted example is not meant to implyarchitectural limitations with respect to the present invention.

FIG. 9 illustrates a high level logic flowchart of a process andcomputer program for optimizing a read data path between one or morememory buffer chips of a memory buffer system operating under aparticular memory protocol and a memory protocol agnostic host employingan HSS interface.

As illustrated, in one example, a process and computer program begin atblock 900 and thereafter proceed to block 902. Block 902 illustratesstarting one or more AC clocks, such as a clock of AC 230 in FIG. 2.Next, block 904 illustrates sending a BCLK signal to the DC, such as DCset 220 and DC set 222 in FIG. 2. Thereafter, block 906 illustratesestablishing the BCOM signal from the AC to each DC, such as to of DCset 220 and DC set 222. In one example, the processes described in block902, block 904, and block 906 may be performed concurrently.

Block 908 illustrates training an AC HSS link, but skipping DC HSStraining. Next, block 910 illustrates using the AC HSS link and BCOM formemory interface training. Thereafter, block 912 illustrates determininga latest strobe arrival time, and the process passes to block 914.

Block 914 illustrates a determination whether DC chip clock externalfeedback is required. At block 914, if no DC chip clock externalfeedback is required, then the process passes to block 916. Block 916illustrates adjusting a DC chip clock phase to align to the lateststrobe. Next, block 918 illustrates adjusting the DC chip WR clock phaserotators, and the process passes to block 920.

Returning to block 914, if DC chip clock external feedback is required,then the process passes either passes to block 926, for theconfiguration illustrated in FIG. 4 or passes to block 928, for theconfiguration illustrated in FIG. 6. Block 926 illustrates implementingthe PLL feedback path with matching delay, and the process passes toblock 916. Alternatively, block 928 illustrates applying a 180-degreephase alignment with the L1 and L2 latches, and the process passes toblock 920.

Block 920 illustrates running DC HSS training. Next, block 922illustrates using the newly adjusted DC chip clock phase to tune the HSSclock to launch the unload pointer, and the process ends.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, occur substantiallyconcurrently, or the blocks may sometimes occur in the reverse order,depending upon the functionality involved. It will also be noted thateach block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising”, when used in this specification specify thepresence of stated features, integers, steps, operations, elements,and/or components, but not preclude the presence or addition of one ormore other features, integers, steps, operations, elements, components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the one or more embodiments of the invention has beenpresented for purposes of illustration and description, but is notintended to be exhaustive or limited to the invention in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art without departing from the scope and spiritof the invention. The embodiment was chosen and described to bestexplain the principles of the invention and the practical application,and to enable others of ordinary skill in the art to understand theinvention for various embodiments with various modifications as aresuited to the particular use contemplated.

While the invention has been particularly shown and described withreference to one or more embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method comprising: determining, by a computersystem, a latest arriving data strobe from at least one data chip at afirst data buffer in a read data path between at least one memory chipand a host on a high speed interface; determining, by the computersystem, whether external feedback of the at least one data chip isrequired; and in response to determining that external feedback of theat least one data chip is required, aligning, by the computer system, achip clock distributed to a second data buffer in the read data pathwith the latest arriving data strobe by applying a 180 degree phasealign of the chip clock through one or more latches, wherein data crossa first clock boundary from the first data buffer to the second databuffer, to minimize a latency in the read data path across the firstclock boundary.
 2. The method according to claim 1, further comprising:starting, by the computer system, a buffer clock by an address chip of amemory buffer system comprising the at least one memory chip; sending,by the computer system, the buffer clock signal to at least one datachip of the memory buffer system, the buffer clock driving a phaselocked loop of the at least one data chip, which drives the chip clock;establishing, by the computer system, a broadcast command signal fromthe address chip to the at least one data chip; training, by thecomputer system, a link between the address chip and the read data path;training, by the computer system, a memory interface of the memorybuffer system to enable read and write data paths from the at least onedata chip; determining, by the computer system, an arrival time of thelatest arriving data strobe from the at least one data chip; anddetermining, by the computer system, whether external feedback of the atleast one data chip is required.
 3. The method according to claim 2,further comprising: in response to determining external feedback of theat least one data chip is not required, adjusting, by the computersystem, a phase of the chip clock to align to a phase of the latestarriving strobe; and adjusting, by the computer system, one or morephases of a write clock of the data chip.
 4. The method according toclaim 2, further comprising: in response to determining externalfeedback of the at least one data chip is required, implementing, by thecomputer system, a phase locked loop feedback path with equal delay toadjust the phase lock loop phase; adjusting, by the computer system, aphase of the chip clock to align to a phase of the latest arrivingstrobe; and adjusting, by the computer system, one or more phases of awrite clock of the data chip.
 5. The method according to claim 1,further comprising: aligning, by the computer system, the chip clockwith a high speed clock for controlling an unload pointer to unload thedata from the second data buffer to a serializer in the read data path,wherein the data cross a second clock boundary from the second databuffer to the serializer, to minimize a latency in the read data pathacross a second clock boundary by: training, by the computer system, alink between the at least one data chip and the serializer; and using,by the computer system, the chip clock, aligned by the latest arrivingdata strobe, to tune the launch clock to minimize a number of clockcycles between the data loading into the second data buffer and the datacrossing the second clock boundary to the serializer in response to thehigh speed clock.
 6. The method according to claim 1, furthercomprising: applying, by the computer system, the 180 degree phase alignof an alternate edge of the chip clock through one or more latches forunloading the data on one of the rising edge and falling edge of the oneor more latches.
 7. The method according to claim 1, further comprising:training, by the computer system, the read data path for minimizinglatency along the read data path for a read tag response from the memorychip to an asynchronous read request from the host.
 8. The methodaccording to claim 1, wherein determining, by a computer system, alatest arriving data strobe from at least one data chip at a first databuffer in a read data path between at least one memory chip and a hoston a high speed interface further comprises: determining, by thecomputer system, the latest arriving data strobe at the first databuffer in a read data path between the at least one memory chipimplementing a synchronous double data rate protocol and the host whichis agnostic to the synchronous double data rate protocol, wherein thehigh speed interface is a SerDes based high speed interface, wherein aread data return on the read data path transmits the data outside theboundaries of the synchronous double data rate protocol.
 9. A computersystem comprising one or more processors, one or more computer-readablememories, one or more computer-readable storage devices, and programinstructions, stored on at least one of the one or more storage devicesfor execution by at least one of the one or more processors via at leastone of the one or more memories, the stored program instructionscomprising: program instructions to determine a latest arriving datastrobe from at least one data chip at a first data buffer in a read datapath between at least one memory chip and a host on a high speedinterface; program instructions to determine whether external feedbackof the at least one data chip is required; and program instructions, inresponse to determining that external feedback of the at least one datachip is required, to align a chip clock distributed to a second databuffer in the read data path with the latest arriving data strobe byapplying a 180 degree phase align of the chip clock through one or morelatches, wherein data cross a first clock boundary from the first databuffer to the second data buffer, to minimize a latency in the read datapath across the first clock boundary.
 10. The computer system accordingto claim 9, the stored program instructions further comprising: programinstructions to start a buffer clock by an address chip of a memorybuffer system comprising the at least one memory chip; programinstructions to send the buffer clock signal to at least one data chipof the memory buffer system, the buffer clock driving a phase lockedloop of the at least one data chip, which drives the chip clock; programinstructions to establish a broadcast command signal from the addresschip to the at least one data chip; program instructions to train a linkbetween the address chip and the read data path; program instructions totrain a memory interface of the memory buffer system to enable read andwrite data paths from the at least one data chip; program instructionsto determine an arrival time of the latest arriving data strobe from theat least one data chip; and program instructions to determine whetherexternal feedback of the at least one data chip is required.
 11. Thecomputer system according to claim 10, the stored program instructionsfurther comprising: program instructions to, in response to determiningexternal feedback of the at least one data chip is not required, adjusta phase of the chip clock to align to a phase of the latest arrivingstrobe; and program instructions to adjust one or more phases of a writeclock of the data chip.
 12. The computer system according to claim 10,the stored program instructions further comprising: program instructionsto, in response to determining external feedback of the at least onedata chip is required, implement a phase locked loop feedback path withequal delay to adjust the phase lock loop phase; program instructions toadjust a phase of the chip clock to align to a phase of the latestarriving strobe; and program instructions to adjust one or more phasesof a write clock of the data chip.
 13. The computer system according toclaim 9, the stored program instructions further comprising: programinstructions to align the chip clock with a high speed clock forcontrolling an unload pointer to unload the data from the second databuffer to a serializer in the read data path, wherein the data cross asecond clock boundary from the second data buffer to the serializer, tominimize a latency in the read data path across a second clock boundaryfurther comprising: program instructions to train a link between the atleast one data chip and the serializer; and program instructions to usethe chip clock, aligned by the latest arriving data strobe, to tune thelaunch clock to minimize a number of clock cycles between the dataloading into the second data buffer and the data crossing the secondclock boundary to the serializer in response to the high speed clock.14. The computer system according to claim 9, the stored programinstructions further comprising: program instructions to apply the 180degree phase align of an alternate edge of the chip clock through one ormore latches for unloading the data on one of the rising edge andfalling edge of the one or more latches.
 15. The computer systemaccording to claim 9, the stored program instructions furthercomprising: program instructions to train the read data path forminimizing latency along the read data path for a read tag response fromthe memory chip to an asynchronous read request from the host.
 16. Thecomputer system according to claim 9, wherein the program instructionsto determine a latest arriving data strobe from at least one data chipat a first data buffer in a read data path between at least one memorychip and a host on a high speed interface further comprise: programinstructions to determine the latest arriving data strobe at the firstdata buffer in a read data path between the at least one memory chipimplementing a synchronous double data rate protocol and the host whichis agnostic to the synchronous double data rate protocol, wherein thehigh speed interface is a SerDes based high speed interface, wherein aread data return on the read data path transmits the data outside theboundaries of the synchronous double data rate protocol.
 17. A computerprogram product comprising one or more computer-readable storage devicesand program instructions, stored on at least one of the one or morestorage devices, the stored program instructions comprising: programinstructions to determine a latest arriving data strobe from at leastone data chip at a first data buffer in a read data path between atleast one memory chip and a host on a high speed interface; programinstructions to determine whether external feedback of the at least onedata chip is required; and program instructions, in response todetermining that external feedback of the at least one data chip isrequired, to align a chip clock distributed to a second data buffer inthe read data path with the latest arriving data strobe by applying a180 degree phase align of the chip clock through one or more latches,wherein data cross a first clock boundary from the first data buffer tothe second data buffer, to minimize a latency in the read data pathacross the first clock boundary.
 18. The computer program productaccording to claim 17, the stored program instructions furthercomprising: program instructions to start a buffer clock by an addresschip of a memory buffer system comprising the at least one memory chip;program instructions to send the buffer clock signal to at least onedata chip of the memory buffer system, the buffer clock driving a phaselocked loop of the at least one data chip, which drives the chip clock;program instructions to establish a broadcast command signal from theaddress chip to the at least one data chip; program instructions totrain a link between the address chip and the read data path; programinstructions to train a memory interface of the memory buffer system toenable read and write data paths from the at least one data chip;program instructions to determine an arrival time of the latest arrivingdata strobe from the at least one data chip; and program instructions todetermine whether external feedback of the at least one data chip isrequired.
 19. The computer program product according to claim 17, thestored program instructions further comprising: program instructions to,in response to determining external feedback of the at least one datachip is not required, adjust a phase of the chip clock to align to aphase of the latest arriving strobe; and program instructions to adjustone or more phases of a write clock of the data chip.
 20. The computerprogram product according to claim 17, the stored program instructionsfurther comprising: program instructions to, in response to determiningexternal feedback of the at least one data chip is required, implement aphase locked loop feedback path with equal delay to adjust the phaselock loop phase; program instructions to adjust a phase of the chipclock to align to a phase of the latest arriving strobe; and programinstructions to adjust one or more phases of a write clock of the datachip.